One of the inherent features of modern internal combustion (IC) engines is that their operating cycle includes a compression stroke, in which a mixture comprising air and a hydrocarbon or other fuel is compressed in a cylindrical volume prior to its ignition, from which ignition is developed a force that is advantageously used to propel one or more pistons. In a typical internal combustion engine, one or more pistons are connected to a crankshaft that is rotably disposed in an engine block, and the repeated, controlled ignition of compressed air/fuel mixtures serve as the main motive means of propulsion for causing rotation of the engine's crankshaft.
By its nature, the combustion of compressed air/fuel mixtures permits some degree of freedom in engineering selection of the degree to which the air/fuel mixture to be combusted is compressed. Compression ratio is used to refer to the degree which an air/fuel mixture is compressed prior to its combustion. Generally, there is a mechanical compression ratio, which represents the theoretical maximum possible compression ratio. The mechanical compression ratio is derived from the ratio of the combined volume within the cylinder and combustion chamber in which a piston is disposed when the piston is at its bottom dead center (“BDC”) position in a cylinder bore, versus the combined volumes within the same cylinder and combustion chamber when the piston is at its top dead center (“TDC”) position. Mechanical compression ratios on the order of between about 8:1 to about 12:1 have been employed in spark-ignition engines employing gasolines as a fuel.
The effective compression ratio or dynamic compression ratio, on the other hand, is often somewhat less than the mechanical compression ratio. This discrepancy is due, among other things, to such factors as the volumetric efficiency of an IC engine, valve timing, engine r.p.m., air temperature, and inertial forces inherent in a flowing mass of air/fuel mixture. Effective compression ratios on the order of between about 7:1 to about 10:1 have been achieved in IC engines.
A typical IC engine includes a valve train, which includes a camshaft that by virtue of its rotation causes the opening and closing of one or more intake and exhaust valves associated with a cylinder on the IC engine. Camshafts may be rotably disposed within an engine block, or may be rotably mounted atop or in a cylinder head. Engine configurations having a single camshaft to actuate both intake and exhaust valves are known, as are engines which employ a single camshaft for operating one or more intake valves only, with a separate camshaft being used to operate one or more exhaust valves only. In any event, the rotation of the camshaft(s) are often mechanically coupled to the rotation of the engine's crankshaft, typically through a timing chain or by means of one or more timing belts. It is generally desirable for an engine's valve timing to have both intake and exhaust valves substantially closed during the engines compression stroke, to enable compression of the air/fuel mixture present in the cylinder under consideration. Modifications to camshaft profiles have been made which sometimes provide for an intake valve to not be completely closed when the piston begins its upward travel in a cylinder during a compression stroke. Additionally, some camshafts exist which permit for a cylinder's exhaust valve to remain open after the piston has reached TDC on an exhaust stroke. The selection by a camshaft designer of preferred precise timing events such as the angular position of the crankshaft within the engine block expressed as degrees of rotation of the crankshaft during a single engine cycle when an intake valve or exhaust valve begins to open or close is fairly specific to a given engine design, taking into account all relevant factors relating to the engine's design, intended use, and anticipated service range of r.p.m. Cam timing events include intake and exhaust valve opening and closing events, and are typically expressed as occurring using degrees of crankshaft rotation prior to the event. In some embodiments, these are expressed at zero lift, and in alternate embodiments these may be expressed at any desired specified amount of valve lift off its corresponding seat. Additionally, cam centerline, lobe separation, valve lift, duration as measured at a specified amount of lift, overlap, and valve timing advance are other variables at the disposal of a camshaft designer and automotive engineer.
Given the variety of engine sizes and end-use requirements for IC engines, it has not been possible to this day to provide a single camshaft profile that is suitable for the needs of all IC engines; accordingly, a wide range of camshaft profiles and timing events are in popular use. Often, camshaft grinds which are optimal for high power output perform poorly with respect to fuel consumption, and camshaft grinds which perform well respecting fuel consumption do not deliver the maximum possible torque output for a given engine configuration. Thus, a camshaft selected for a given engine design and configuration often represents a compromise between these two factors. The trade-off between power and economy mentioned above and other constraints including engine emissions, have provided some limitations on the versatility of the operation of engines, particularly those operated using fuels of inconsistent chemical composition over the useful service life of a given engine.